We have engineered immunotoxins into exquisitely cell type specific reagents with promise for cancer therapy. Exploring their applications in vivo we have found that 1) there are powerful pharmacologic barriers that limit protein access to tumor cells; 2) this problem is exacerbated in the brain where the blood-brain barrier prevents macromolecule movement into the brain tissue; 3) the plant and bacterial toxins used for construction of immunotoxins are highly immunogenic and soon after treatment antibodies arise that inactivate reagent.Thus, to overcome the problems of delivery and immunogenicity, we have pursued regional delivery of immunotoxins to the brain as away to treat brain tumors. Since cancer can spread and grow in the CSF, a condition known as leptomeningeal carcinomatosis, immunotoxins were initially injected directly into the cerebral spinal fluid to access tumor cells and were found to kill 99% to 99.9% of the tumor cells in vivo with occasional animals cured. An intriguing dose limiting toxicity was found specifically related to this route of administration. Purkinje cells were killed by diptheria toxin derived immunotoxin guinea pigs and ricin derived immunotoxins in rats and monkeys. Another protein, called the eosinophil-derived neurotoxin is homologous to RNases A and also selectively kills Purkinje cells. 4) Comparing a family of homologous RNases we found 5000-fold variation in cytotoxicity. The molecular basis of toxicity was explored and cell binding, RNase inhibitor sensitivity and/or enzyme activity all appear to contribute. 7) We have determined the dose limiting toxicity of immunotoxins in three model species, guinea pigs, rats and rhesus monkeys and in man. 8) The pharmacology of a monoclonal antibody against the transferrin receptor, 454A12, coupled to recombinant ricin A chain was thoroughly studied in primates and man. Clearance from the CSF was biphasic and in humans, a somewhat larger clearance rate was found for the antitransferrin receptor immunotoxin than seen with other macromolecules possibly reflecting uptake by tumor cells. A potentially large therapeutic window exists for intrathecal immunotoxins for cancer therapy.